This invention relates to a wet-spinning process for making acrylonitrile polymer filaments.
Wet-spinning processes for production of acrylonitrilepolymer filaments are well known. See, for example, U.S. Pat. Nos. 3,088,188 and 3,402,235, the disclosures of which are incorporated hereinby reference. In such processes, an acrylonitrile polymer solution is extruded through a spinnerette submerged in a coagulant bath which generally comprises water and solvent. The solvent in the extruded stream of polymer solution migrates into the bath and solidification of the polymer in filament form results. The polymer is generally stretched (drawn) while still in the coagulation bath to at least partially collapse voids resulting from interchange of polymer solvent and bath liquid. Normally, during coagulation there is an inward diffusion of coagulation bath liquid into the filaments being coagulated and a corresponding outward movement of polymer solvent into the coagulation bath. The solvent and the bath liquid in most prior commercial processes interchange in such a manner that the resulting filaments contain voids or cavities along their lengths which can be seen clearly with an optical phase microscope. Filaments containing these voids or unfilled spaces do not possess the requisite physical properties for some end uses. For example, such filaments exhibit a delustered appearance, lower tenacity, and lower abrasion resistance as compared with filaments not containing voids.
To overcome this physical weakness inherently formed in the filaments, positive after treatment steps during the processing of the filaments normally are taken. The tenacity of the filaments is greatly improved by stretching to molecularly orient the polymer molecules and at least partially collapse these voids. To more fully collapse these voids the filaments may be dried at rather high temperatures under tension, thereby forming a more dense filamentary structure. The tenacity of the filaments is generally satisfactory with such after treatment. However, tenacity is primarily a longitudinal property of the filaments; and satisfactory tenacity, alone, is not the full answer to the attainment of filaments having an optimum balance of properties. In many end uses, the abrasion resistance and the resistance to break upon being flexed (flex life) are highly important. Such properties may be regarded as lateral properties as distinguished from longitudinal properties. While drying under tension gives the illusion of forming filaments without voids, the voids are merely collapsed. Although the collapsed voids do not detract from the longitudinal properties of the filaments to any significant extent, it has been found that lateral stresses cause filaments to splinter or break. In other words, filaments having voids which are merely collapsed are laterally weak.
In addition to the possible presence of the voids which are visible under an optical phase microscope, electron microscopy has shown the existence of a reticulate structure in the filaments displaying a network of submicroscopic pores or interstitial spaces most of which intercommunicate with each other. The pores in freshly spun filaments, that is filaments which have been coagulated without having been subjected to any after treatment producing a pronounced change in the structure, are readily observable under an electron microscope. The polymers comprising the filaments appear to take the form of a latticework of integrally joined strings. The polymer lattice has a pattern resembling that of a fine, extremely small meshwork, although the interstices are usually somewhat irregular in size and shape. The micropores present in filaments produced by ordinary wet-spinning techniques as they leave the coagulating bath are more or less spherical. The distances across these spaces are ordinarily about 250 A. to 3000 A. or greater. The frequency of occurrence of the micropores in the filaments produced by ordinary wet-spinning techniques employing aqueous coagulating baths can be estimated under an electron microscope and is usually 35-90.times.10.sup.14 per gram of polymer.
The art has found that the properties of the filaments can be improved substantially by subjecting the filaments to an annealing operation. Annealing can be accomplished by placing the acrylonitrile polymer filaments in a closed chamber, subjecting them to a high temperature and pressure in the presence of wet steam and then evacuating the chamber. This treating cycle is repeated as many times as needed. It will be appreciated that this annealing operation is expensive and time consuming. However, omitting the annealing step in the after treatment of conventionally wet spun acrylic filaments results in filaments having a tendency to splinter or fibrillate; and hence, the filaments have a low abrasion resistance.
Prior to this invention, processes using organic, nonsalt based solvents other than processes using organic solvent systems undesirable for commercial processes did not permit obtaining products with acceptable properties without a high pressure steam annealing step.
It will be appreciated by those skilled in the art that an improved wet-spinning process capable of providing filaments which do not require batch annealing to obtain acceptable properties would constitute a substantive technical achievement.